Although the present invention has general applicability to any given aqueous system where corrosion of steel is a potential problem, the invention will be discussed in detail as it concerns cooling water and boiling water.
The term "cooling water" is applied whenever water is circulated through equipment to absorb and carry away heat. This definition includes air conditioning systems, engine jacket systems, refrigeration systems as well as the multitude of industrial heat exchange operations, such as found in oil refineries, chemical plants, steel mills, etc.
The use of recirculating systems in which a cooling tower, spray pond, evaporative condenser, and the like serve to dissipate heat permits great economy in makeup water requirements. With the increased awareness of the environmental affects of once-through water systems, increasing use is made of recirculating systems in which water is used over and over again.
For example, after passage of circulating cooling water through heat exchange equipment, the water is cooled when passed through a cooling tower. This cooling effect is produced by evaporation of a portion of the circulating water in passing through the cooling tower. By virtue of the evaporation which takes place in cooling, the dissolved solids and the suspended solids in the water become concentrated.
The circulating water becomes more concentrated than the makeup water due to this evaporation loss. Cycles of concentration is the term employed to indicate the degree of concentration of the circulating water as compared with the makeup water. For example, two cycles of concentration indicates the circulating water has twice the concentration of ions as the makeup water. The use of oxidizers which promote passivation, such as chromate, nitrite, molybdate and tungstate is known in both once through and recirculating cooling water systems. See Betz Handbook of Industrial Water Conditioning, 1980, pages 173-174.
As described comprehensively in U.S. Pat. No. 4,288,327, the deposition of solids onto heat transfer surfaces of steam generating equipment is a major problem. Common contaminants in boiler feedwater that can form deposits are calcium and magnesium salts (hardness), carbonate salts, sulfite, phosphate, siliceous matter, and iron oxides. Any foreign matter introduced into a boiler in soluble or particulate form will tend to form deposits on the heat transfer surfaces. Formation of deposits on the transfer surfaces takes place and can lead to overheating, circulation restrictions, damage to the system, loss of effectiveness and increased cost due to cleaning, unscheduled outages and replacement of equipment. Also, such deposits can aggravate corrosion of the underlying metal.
Deposits in lines, heat exchange equipment etc., may originate from several causes. For example, the precipitation of calcium salts will form scale. In addition, solid foulant particles may enter the system and through collision with neighboring solid particles, these foulants may agglomerate to a point where they either foul the heat transfer surfaces or begin to accumulate in low flow areas of the system. On the other hand, corrosion may occur. Corrosion is the electrochemical reaction of metal with its environment. It is a destructive reaction and, simply stated, is the reversion of refined metals to their natural state. Also, concomitant with the corrosion process, hydrogen attack or embrittlement can occur where hydrogen permeates the metal structure, reacting with iron carbide to form methane which results in rupture along the crystalline boundaries.
In the past, in order to minimize the formation of scale forming salts, cooling water systems were operated at pH's where the solubility of the "hardness" or "scale forming" ions was the greatest. Because the pH's of the systems were acidic, corrosion inhibitors together with dispersants were the normal treatment. These materials interacted with the metal to directly produce a film which was resistant to corrosion, or to indirectly promote formation of protective films by activating the metal surface so as to form a stable oxide or other insoluble salt. However, such protective films are not completely stable, but rather constantly degrading under the influences of the aggressive conditions in the water. Because of this, a constant supply of corrosion inhibiting substances, sufficient for the purpose, must be maintained in the water.
Similarly, the formation of scale and sludge deposits on boiler heating surfaces is a serious problem encountered in steam generation. Although current industrial steam producing systems make use of sophisticated external treatment of the boiler feedwater, e.g. coagulation, filtration, softening of water prior to its feed into the boiler system, these operations are only moderately effective. In all cases, external treatment does not in itself provide adequate treatment since muds, sludge, silts and hardness imparting ions can escape the treatment and eventually are introduced into the steam generating system. Accordingly, internal treatments have been necessary to maintain the mud and silts in a suspended state. These internal treatments have been referred to in the industry as sludge conditioning agents.
In addition to the problems caused by mud, sludge, or silts, the industry has also had to contend with scale in boiler and cooling water. Although external treatment is utilized specifically in an attempt to remove calcium and magnesium from the feedwater, scale formation due to residual hardness, i.e., calcium and magnesium salts, is always experienced. Accordingly, internal treatment, i.e., treatment of the water fed to the system, is necessary to prevent, reduce, and or retard formation of scale imparting compounds and their deposition. As in cooling water, pH is employed to control deposition and also corrosion in boiler systems.
Many and different types of materials have been used in the treatment of water systems. For example, corrosion in a boiler condensate system may be treated by mechanical deaeration of the feedwater and chemical oxygen scavenging. Neutralizing and filming amines are also commonly employed in boiler water condensate systems to control corrosion. In open recirculating cooling water systems, corrosion control is primarily achieved by additives which retard destruction of metals by chemical or electrochemical reactions. For example, combinations of chromate, polyphosphate, and zinc are well known cooling system corrosion inhibitors. U.S. Pat. No. 4,446,045 discusses a number of deposition and scale control materials used for the treatment of water systems.